OVERVIEWIntroductionSources Of Ground WaterDischarge Of Ground WaterWater Table AquifersGround HydraulicsPumping Tests

Introduction

Water that collects or flows beneath the earth’s surface, filling the porous spaces in soil ,sediment, and rocks. Ground water originates from melting of snow and ice and is the source of water for aquifers, springs and wells.

Sources of Ground waterPrecipitation seeping through Pores and

cracks in ground Rain and melted snow soak.

Types of ground water There are three main types of ground water.

Meteoric WaterConnate WaterJuvenile Water

Meteoric water Most groundwater is meteoric water. Other

forms normally do not play a significant role in the hydrologic cycle. Meteoric water is a hydrological term for long term standing water in the ground.

Non meteoric water Non-meteoric forms of groundwater are 

connate water and magmatic water, also termed juvenile water.

Connate water is trapped in rock stratum at the time of formation. Because rock containing connate water is typically formed from ocean sediments, connate water is normally saline.

Magmatic water rises from great depth accompanying magma intrusion and affects mineralogy

Subsurface zones Two major sub surface zones divided by a

boundary called water table.

Vadose zone (zone of aeration) is above the water table in which soil pores may either contain air or water.

Phreatic zone(zone of saturation) is below the water table where the pores are filled with water.

Vadose zone Three moisture regions in vadose zone.

Soil water region; ranging 10 m below soil surface, having roots of vegetation etc. this region fluctuates in amount due to moisture uptake from vegetation.

Intermediet region; is a region between soil water region and capillary fringe. Here the moisture levels remain constant at field capacity of soil and rocks of the region.

Capillary fringe; is the region just above the water table where moisture rises due to capillary action.

Phreatic zoneThe phreatic zone is the area in an aquifer

below the water table in which relatively all the pores and fractures are saturated with water. The phreatic zone may fluctuate with changes of season and during wet and dry periods.

Subsurface zones

Water TableThe water table is the level at which the

submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity.

The water table is the locus of point (in unconfined material) where hydrostatic pressure equals atmospheric pressure.

AquiferA geologic formation which contains water

and transmits it from one point to another in quantities sufficient to permit economic development is called an aquifer.

Types of Aquifers1- Confined Aquifer

2- Unconfined Aquifer

Aquifers

Confined aquifer An aquifer which is sandwiched between two layers

of less permeable material is called confined aquifer.

A confined aquifer is a water-bearing stratum that is confined or overlain by a rock layer that does not transmit water in any appreciable amount or that is impermeable. There probably are few truly confined aquifers, because tests have shown that the confining strata, or layers, although they do not readily transmit water, over a period of time contribute large quantities of water by slow.

Unconfined aquiferAn unconfined aquifer is one that is open to

receive water from the surface, and whose water table surface is free to fluctuate up and down, depending on the recharge/discharge rate.  There are no overlying "confining beds" of low permeability to physically isolate the groundwater system.

LEAKY AQUIFERSThe levels to which the water from the confined aquifer

rises define the piezometric surface. The potentiometric surface can be under the top of the

semi-pervious layer, above it but under the water table of the unconfined aquifer overlying it, or finally above the water table.

A special case is when the potentiometric surface is above the ground level, the well penetrating the confined aquifer being artesian. The differences in hydraulic head between the unconfined and the confined aquifer lead to vertical fluxes directed from the aquifer having the upper value to the aquifer with the lower value of the hydraulic head. The vertical water transfer is called leakage and will thus be directed upward or downward

Semi confined / Leaky aquifers

RECHARGE IN UNCONFINED AQUIFERS

Natural recharge of the unconfined aquifers is mainly due to the downward seepage (or percolation) through the unsaturated zone of the excess water over passing the field capacity of the soil. Recharge can also occur through upward seepage (leakage) from underlying aquifers

Water Divide

When two rivers drain an unconfined aquifer, an underground water divide corresponding to the crest of the water table separates the drainage areas of each river

Recharge In Confined AquiferA regional confined aquifer is directly

recharged by precipitation in the area where the aquifer crops out, having the same characteristics as an unconfined aquifer, Another source of recharge is the infiltration in the same cropping out area of the runoff born on the hillslopes during rains or snowmelt .

Recharge in Leaky Aquifers

The confined aquifers are recharged by direct infiltration and percolation or by leakage. In the case of a multi-strata aquifer, depending on the value of the water abstraction, a confined layer can be recharged by vertical fluxes coming from the overlying and underlying aquifers.

The confined aquifer denoted by 1 is characterized by the hydraulic head H1, where H2 is the hydraulic head of the second confined aquifer; thus there will be a flux directed from the confined aquifer no.2 toward the aquifer no.1. At the same time, the hydraulic head H1 is under the water table of the unconfined aquifer, which explains the downward flux from the unconfined aquifer to the first confined aquifer.

DrawdownWhen water is

pumped from a well the water level in the well falls. This fall is called drawdown. The amount of water that can be pumped is limited by the drawdown produced. Typically, drawdown also increases with the length of time that the pumping continues

Storativity, specific storage….Specific storage (Ss), storativity (S), specific

yield (Sy) and specific capacity are material physical properties that characterize the capacity of an aquifer to release groundwater from storage in response to a decline in hydraulic head.

For that reason they are sometimes referred to as "storage properties". In the field of hydrogeology, these properties are often determined using some combination of field hydraulic tests (e.g., aquifer tests) and laboratory tests on aquifer material samples

Storage/storativityStorativity is the volume of water released from storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer, or:This is a measure of the water stored and released in an aquifer and is used to quantify the safe yield of an aquifer system

Storativity(S)Storativity is the volume of water released from

storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer, or:

Storativity is the vertically integrated specific storage value for a confined aquifer or aquitard. For a confined homogeneous aquifer or aquitard they are simply related by:

  where b is the thickness of aquifer. Storativity is a dimensionless quantity, and ranges

between 0 and the effective porosity of the aquifer; although for confined aquifers, this number is usually much less than 0.01

The storage coefficient of an unconfined aquifer is approximately equal to the specific yield, Sy, since the release from specific storage, Ss is typically orders of magnitude less

Specific Storage(Ss)In hydrogeology, volumetric specific storage is much

more commonly encountered than mass specific storage. Consequently, the term specific storage generally refers to volumetric specific storage.

It is the volume of water that an aquifer releases from storage, per volume of aquifer, per unit decline in hydraulic head

whereSs is the volumetric specific storage ([L-1]);Va is the bulk volume of that portion of the aquifer from

which the water is released ([L3]);dVw is the volume of water released from storage ([L3]);dp is the decline in pressure(N•m-2 or [ML-1T-2]) ;dh is the decline in hydraulic head ([L]) andγw is the specific weight of water (N•m-3 or [ML-2T-2]).

Specific yields (Sy)Specific yield, also known as the drainable porosity, is

a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:

whereVwd is the volume of water drained, andVT is the total rock or material volume It is primarily used for unconfined aquifers, since the

elastic storage component, Ss, is relatively small and usually has an insignificant contribution. Specific yield can be close to effective porosity, but there are several subtle things which make this value more complicated than it seems. Some water always remains in the formation, even after drainage; it clings to the grains of sand and clay in the formation. Also, the value of specific yield may not be fully realized for a very long time, due to complications caused by unsaturated flow.

Specific yieldMaterial

Specific Yield (%)

min avg max

Unconsolidated deposits

Clay 0 2 5

Sandy clay (mud) 3 7 12

Silt 3 18 19

Fine sand 10 21 28

Medium sand 15 26 32

Coarse sand 20 27 35

Gravelly sand 20 25 35

Fine gravel 21 25 35

Medium gravel 13 23 26

Coarse gravel 12 22 26

Consolidated deposits

Fine-grained sandstone   21  

Medium-grained sandstone   27  

Limestone   14  

Schist   26  

Siltstone   12  

Tuff   21  

Other deposits

Dune sand   38  

Loess   18  

Peat   44  

Till, predominantly silt   6  

Till, predominantly sand   16  

Till, predominantly gravel   16  

TransmittivityTransmissivity is the rate at

which water is transmitted through a unit width of an aquifer under a unit hydraulic gradient. It is expressed as the product of the average hydraulic conductivity and thickness of the saturated portion of an aquifer. Transmissivity is a measure of the ease with which groundwater flows in the subsurface

Hydrualic conductivity(K)Hydraulic conductivity, symbolically

represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation. Saturated hydraulic conductivity, Ksat, describes water movement through saturated media.

Hydraulic ConductivitiesUnconsolidated deposits

Hydraulic conductivity(m/s)

RocksHydraulic conductivity

(m/s)

Dense clay 10-13 ……10-8 Dense sandstone 10-9 ……10-7

Weathered clay 10-8 ……10-6 Karstic sandstone 10-7 ……10-5

Silt 10-7 ……10-5 Dense limestone 10-9……10-7

Alluvial deposits 10-5 ……10-3 Karstic limestone 10-5 ……10-3

Fine sand 10-5 ……10-4 Dolomite 10-10 ……10-8

Medium sand 5x10-4 ……5x10-3 Dense crystalline rocks 10-13 ……10-12

Coarse sand 10-4 ……10-3 Fractured crystalline rocks 10-10 ……10-6

Fine gravel 10-3 ……5x10-1 Dense basalt 10-13 ……10-10

Medium gravel 5x10-2 ……10-1 Fractured basalt 10-7 ……10-4

Coarse gravel 10-2 ……5x10-1 Claystone 10-13 ……10-9

Hydraulic head (h)Hydraulic head is the height

above a datum plane such as sea level of the column of water that can be supported by the hydraulic pressure at a given point in a groundwater system. Elevation to which water will rise in a borehole connected to a point in an aquifer under pressure. Hydraulic heads provide an indication of the direction of groundwater flow and is used to determine hydraulic gradients.

Hydraulic gradients and hydraulic headsIf in the flow sense, the contour

lines are distanced, being followed by close contour lines, in the first zone the hydraulic conductivities are greater than in the second zone. The hydraulic gradients being greater in the second region than in the first area, the energy losses are greater, meaning a greater hydraulic resistance or, what is the same thing, smaller hydraulic conductivities than in the first area

Steady state condition

It can be assumed that if water from a well is pumped at a constant flow rate for a long period of time, the cone of depression will have a constant position over time around the well

Aquifer testsConfined aquifer under Steady state

conditions

Unconfined aquifer under steady state conditions

Theim solution

Aquifer testsConfined aquifer under un-Steady flow

conditions

Theis solution cooper jackob method

Unconfined aquifer under un-steady flow conditions

Neuman method solution

Pumping through confined aquifer

Theim solution for steady state confined aquifersTransmittivity

Hydraulic conductivity

Pumping through un-confined aquifer

Theim solution for un-confined aquifersHydraulic conductivity

h1= saturated thickness of aquifer, at radial distance r from pumping well

h2=head in an observation well situated at a distance r2 from the same pumping well

K=hydraulic conductivity Flow rate (Q)

Theis solution for confined aquifers under unsteady conditions

Transmittivity

H0= hydraulic head before pumping start S-S0= s is the drawdown Q=discharge of the well t=time since pumping has started T=transmisivity W(u)= well function or exponential integral and

has the following expression

Cooper Jacob solution for confined aquifers under unsteady conditions

Drawdown

Leaky aquifer with unsteady flow conditions

Hantush and jacob formula

r= distance from the pumping well to the observation wellB=Leakage factort=time since pumping has started The solution for a leaky aquifer will be obtained in the same way as for the

Theis method, with a superposition of drawdown data on top of leaky type curves. A curve of best fit is selected, and values of W, 1/u, s, and t are found. Then the transmisivity T and the storativity S may be determined using the relations stated above. The values of K'and b' may be computed based on the value of r/B:

K’=vertical hydraulic conductivity of aquitardb’=thickness of aquitard

Slug tests A slug test is a particular type of aquifer test where water is

quickly added or removed from a groundwater well, and the change in hydraulic head is monitored through time, to determine the near-well aquifer characteristics. It is a method used by hydrogeologists and civil engineers to determine thetransmissivity/hydraulic conductivity and storativity of the material the well is completed in.

The pumping tests for assesment of the hydraulic conductivity are difficult to be performed for the area where the permeability of the soil is too low. In such area, the slug test method is a viable alternative.

These types of tests allow to determine the hydraulic conductivity in the area around the small diameter monitoring wells.

The principle of the method consists in measuring the rate at which the water level falls or rises in the well after a known volume of water was drawn from or added into the well.

Many methods have been developed to analyze the data from such tests.

SLUG TEST =METHOD 1(confined)

The Cooper - Bredehoeft - Papadopulos method is designed to assess the transmisivity and the storativity for confined aquifers. A known volume of water is suddenly added into the well and as the levels fall the water head and the coresponding time are measured and recorded. Type curves will help then to manage the measured data and to determine the transmisivity and the storativity of the area in the vecinity of the monitoring well (Fetter, 1994).

SLUG TEST –METHOD 2The Hvorslev method uses some piezometers

installed in the aquifer to monitor groundwater levels in which a known volume of water is added or drawn. The water level is measured both before and after the volume of water is added; then it is measured at timed intervals upon which the water level comes back to its initial value. The data obtained from this experiment are plotted on semilogarithmic axes as head ratio H/H0 versus time, that should plot on a straight line.

H0 is the maximum height to which the water level rises above the initial water level (static level);

H is the height of water level above the initial water level at time t.

Hydraulic conductivity by Slug TestIn case the length of the piezometer well is

greater than 8 times the radius, the hydraulic conductivity may be determined by the folowing relation (Fetter, 1994):

SLUG TEST-METHOD 3The Bouwer and Rice method may be used both for unconfined and confined

aquifers. In order to solve the Bouwer and Rice equation of the hydraulic conductivity, some graphs have been plotted and may be found in the literature (Fetter, 1994).

In the Bouwer and Rice method, the water level is lowered so that the water flows from the aquifer into the well or water is added into the well. If the well screen is above the static level, the water from the well will drain into the vadose zone as well as in the saturated area and this will result in an overestimation of the hydraulic conductivity of the saturated area

Slug test in unconfined aquifer